With the advent of plastic food containers a number of lid constructions have been proposed for sealing the contents to keep out ambient air. Difficulties have been encountered in maintaining a tight seal, especially when the closed containers are refrigerated.
U.S. Pat. No. 2,487,400 discloses a flexible plastic cover for an open mouth container, said lid having a recessed top wall with an inverted U shaped rim for fitting over the lip of the container. The lid is described as having elasticity and flexibility with a slow rate of recovery to provide a non-snapping and noiseless type of cover. It is applied to the container by pressing down on the rim progressively along its top wall to effect a spreading of the side groove walls and an expansion or compression of the central wall to effect sealing engagement between the container rim and the inner surfaces of the top and sidewalls of the container rim. Because of the use of plastic material having high flexibility to obtain a non-snapping cover, this construction does not provide a tight seal, especially under refrigerating conditions. Moreover, the recessed top wall of the lid materially reduces the capacity of the container for containing foods.
The Tupper U.S. Pat. No. Re. 24,889 is said to be an improvement over U.S. Pat. No. 2,487,400, and is directed to a rectangular container and cover with rounded corners, the major difference being a bead on the upper rim of the container and inclined inner surfaces on the walls of the peripheral rim groove of the cover forming sealing points with the bead. The cover is of flexible material and is designed to be applied by progressive finger pressure along the top wall of the rim of the cover. This construction has the same disadvantages as the cover of U.S. Pat. No. 2,487,400, including a poor seal and the loss of food capacity of the container due to the recessed top wall of the lid.
The Whitton U.S. Pat. No. 3,111,240 also shows a bead on the container rim which has an inclined surface cooperating with inclined inner surfaces on the interior of a peripheral rim groove to provide secondary seals. However, the primary seal is formed between the smooth inner surface of the container rim and the inner wall of the peripheral rim of the cover and also the recessed lid decreases the capacity of the container.
The Croyle U.S. Pat. No. 3,692,208 discloses a plastic cover having a downwardly concave or conical central wall encircled by an upwardly directed inverted U-shaped annular rim. The inner wall of the rim makes the seal with the inner surface of the bowl rim, thus it is an inner seal as distinguished from the outer seals of the Tupper patents, and there is no sealing contact between the outer surface of the bowl rim and the outer wall of the rim. The downwardly concave central wall of the cover when pressed downwardly contracts the inner wall of the rim to allow it to pass within the rim of a container, and the material of the cover has an elastic memory to expand when released into sealing contact with the inner surface of the container. However, such a cover tends to shrink radially inward when refrigerated, thus diminishing or weakening the seal, and again there is a loss of capacity of the container due to the recessed lid.